1. Field of the Invention
The present invention relates to an acoustic wave filter device used as, for example, an RF band-pass filter of a cellular phone, and, more particularly, to a longitudinally coupled resonator acoustic wave filter device that utilizes a boundary acoustic wave or a surface acoustic wave.
2. Description of the Related Art
Surface acoustic wave filter devices are widely used as RF band-pass filters of cellular phones.
Japanese Patent Application Publication No. 10-261936 discloses a surface acoustic wave filter device used for an application of this type.
As shown in FIG. 9, in a surface acoustic wave filter device 501 described in Japanese Patent Application Publication No. 10-261936, an electrode structure schematically shown in FIG. 9 is provided on a piezoelectric substrate 502. That is, a pair of input IDTs 503 are arranged in parallel on the piezoelectric substrate 502. Output IDTs 504 are arranged with respect to the IDTs 503 in a surface acoustic wave propagating direction in which a surface acoustic wave propagates.
Reflectors 505 are arranged respectively on both sides of the portion in which one pair of input IDT 503 and output IDT 504 are provided in the surface acoustic wave propagating direction. Similarly, other reflectors 505 are arranged respectively on both sides of the region in which the other pair of input IDT 503 and output IDT 504 are provided in the surface acoustic wave propagating direction. Then, one end of each of the pair of input IDTs 503 is connected in common to an input terminal 506 and the other ends are connected to a ground. One end of each of the pair of output IDTs 504 is connected in common to an output terminal 507, and the other ends are connected to a ground.
In order to reduce a higher transverse mode that causes a spurious response, apodization weights are assigned in the input IDTs 503 and output IDTs 504. In a normal type IDT, the electrode finger overlap width does not vary along both sides in the surface acoustic wave propagating direction. In contrast, apodization weighting is a method of providing weights so that the electrode finger overlap width of an IDT is varied to vary the overlap width in the surface acoustic wave propagating direction.
In the surface acoustic wave filter device described in Japanese Patent Application Publication No. 10-261936, the apodization weights are assigned in the IDTs as described above, so that a higher transverse mode is reduced.
On the other hand, a surface acoustic wave filter device may include narrow pitch electrode finger portions that are located at portions at which IDTs are located adjacent to each other. The interval of electrode fingers of each narrow pitch electrode finger portion is less than that of the remaining portions. In such a surface acoustic wave filter, that is, in a longitudinally coupled resonator surface acoustic wave filter device that utilizes an inter-IDT resonance mode, the transverse mode spurious response is not problematic. This is because the confinement effect as a waveguide is relatively small.
In recent years, various boundary acoustic wave filter devices that utilize a boundary acoustic wave have also been developed. The boundary acoustic wave filter devices have a problem in that, as compared to the surface acoustic wave filter devices, the trapping effect as a waveguide is relatively large and, therefore, a spurious response due to a transverse mode tends to be relatively large. In such boundary acoustic wave filter devices, even when resonance between IDTs that have narrow pitch electrode finger portions are utilized, a spurious response due to a transverse mode tends to be relatively large.
Furthermore, in the surface acoustic wave filter devices and the boundary acoustic wave filter devices, in order to reduce the size, when the overlap width of each IDT is reduced, a transverse mode spurious response is likely to occur.
Thus, even in the surface acoustic wave filter devices that utilize an inter-IDT resonance mode, when the size is reduced, a transverse mode spurious response tends to occur.
Thus, in the surface acoustic wave filter devices and boundary acoustic wave filter devices that utilize an inter-IDT resonance mode, there has not been provided or developed a filter device that can further reduce a spurious response due to a transverse mode with a further reduced loss.